Electronic sensors which are suitable for detecting discontinuities, such as gear teeth, along a target surface are well known and used in automotive applications such as brake systems, cruise control systems, transmission systems, as well as many others. In its environment of use, the sensor apparatus requires a support structure, a sensor housing and means for mounting the sensor housing onto the support structure. The stack-up of tolerances of the components comprising the sensor within the housing in addition to the stack-up of tolerances associated with connection to the mounting bracket for mounting the sensor housing onto the support structure contribute to the difficulty of establishing the precise length of the air gap between the sensor and the target surface. In addition, the location of the sensor and target surface may be hidden and therefore it may be difficult, expensive and impractical to precisely measure. Consequently, there is a need to minimize the stack-up of tolerances.
The majority of magnetic sensors used in automotive applications involve non-adjustable air gap placement, wherein the stack-up of tolerances results in an internal air gap that causes deviation from the optimal external air gap. For example, a rigid bracket is affixed to the body of a magnetic sensor. The magnetic sensor is placed into a sensor bore in the engine block, and the bracket is bolted, via a bolt hole in the bracket, to a threaded mounting hole in a mounting surface of the engine block. When the bracket is bolted, the length of the sensor body from the bolt hole of the bracket to the sensor tip determines the external air gap with respect to the target, which air gap is affected by the stack-up of tolerances. Even though subject to tolerance related placement inaccuracy, this structural mounting methodology is used widely because of the simplicity of the hardware, and ease of assembly and service.
In situations where external air gap variation cannot be tolerated, the external air gap is preset during magnetic sensor installation by means of an adjustable bracket, often referred to as a “side-mount” bracket. The adjustability of side mount brackets resides in a bolt slot, which allows for the bracket to be adjusted along the slot elongation relative to the threaded mounting hole of the mounting surface.
The challenge of maintaining tight air gap tolerances is exacerbated by increased temperatures and alternative materials typically found in automotive environments. Differences in coefficients of thermal expansion and other disparate material properties of elements of a sensor mounting assembly can adversely impact system performance. For example, certain materials can exhibit plastic behavior when subjected to tight clamping forces at high temperatures. Thus, the integrity of the mounting of a sensor can be lost under harsh conditions, resulting in looseness, misalignment and even catastrophic failure.
Certain solutions have been proposed, including the use of composite materials and/or reinforcing high strength inserts within base materials having low yield strength. For example, annular metal bushings are often inserted within through passages for fasteners and affixed to the base material through bonding or the use of knurls on the outer peripheral surface of the insert. When torque is applied, the fastener extending through the bushing effects compressive loading to the bushing, rather than the surrounding base material and, if properly designed, avoids exceeding the yield point of the base material. A significant disadvantage of this approach is in the tendency of the annular bushing to lose adhesion or affixation with the surrounding base material and rotate with the fastener. This results in a loose sensor mounting.
What remains needed in the art, is a robust sensor mounting design which tolerates the use of non-traditional, low cost materials while maintaining precision of alignment and system integrity under a full range of harsh operating conditions.